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What Are Glands?
In simple terms, glands are organs that synthesize substances for release. There are two different types of glands – endocrine glands and exocrine glands. The main distinction between the two types of glands is where they secrete their substances. Endocrine glands secrete their substances into the bloodstream, while exocrine glands release substances onto epithelial surfaces (Figure 1).
Examples of Endocrine Glands
There are various endocrine glands throughout the body that secrete different types of hormones into the bloodstream. For example, the pancreas is an endocrine gland that secretes insulin and glucagon into the bloodstream to help regulate blood glucose concentrations. The thyroid gland releases thyroid hormone, which regulates basal metabolic rate. Similarly, the parathyroid glands secrete parathyroid hormone, which helps regulate blood calcium concentrations. The hypothalamus and pituitary gland secrete a variety of different hormones important for physiology, while the ovaries and testes secrete hormones important for reproductive processes.
Examples of Exocrine Glands
Exocrine glands secrete substances onto epithelial surfaces. One example of an epithelial surface is the skin. The sweat glands and sebaceous glands secrete sweat and oil onto the surface of the skin, respectively. Also, the mammary glands secrete milk, which is released onto the skin surface. Epithelial surfaces can also include surfaces within the body. One example is the gastrointestinal tract. Although the pancreas is an endocrine gland, it also has exocrine functions as well (Figure 2). It secretes digestive enzymes onto the surface of the gastrointestinal tract. The gastric glands secrete acid, pepsinogen, and mucus onto the stomach and other components of the gastrointestinal tract. Furthermore, the salivary glands secrete saliva into the mouth, which is also part of the gastrointestinal tract. Another gland, the prostate gland, secretes a component of semen that passes through the urinary tract in males.
